phing: add README (#8278)

plugins/phing/README.md

+# Phing plugin
+
+This plugin adds autocompletion for [`phing`](https://github.com/phingofficial/phing) targets.
+
+To use it, add `phing` to the plugins array of your `.zshrc` file:
+```
+plugins=(... eecms)
+```